Cavities are often filled with a material for insulation or other purposes. In one instance this can for example be a tank with double walls where the cavity between the walls is filled with cement or other hardening material. In another instance it can be a special purpose building, for example a power station having walls where the cavity is filled with cement. Some times it may be necessary to ascertain the quality of the filling but where there are difficulties due to inaccessibility or safety reasons.
One typical example of such a cavity is the annular space between the casing strings of a hydrocarbon well. A typical hydrocarbon well construction consists of a large diameter pipe called a conductor and within that successive pipes called casing strings that are installed in the well as the drilling progresses. When drilling the well first a shallow hole (˜100 meters) is drilled and a pipe, called conductor pipe, is lowered into the hole. Cement is then pumped down the pipe and allowed to flow up in the space between the conductor pipe and the surrounding ground. Then the hole is drilled further down and a second casing, called surface casing and normally 20 inches nominal diameter, is installed in the well and the annular space between the surface casing and the borehole (in the open part of the hole) and the conductor pipe is also filled with cement. Then, depending on the length of the hole drilled, and the rock structure, successive casing strings with diminishing diameters are introduced into the borehole and hung off from the wellhead. These casings are normally cemented only partway up from the bottom of the borehole. Lastly, production tubing is installed into the well down to the producing formation and the casings are perforated to allow fluids to enter the well to flow up through the tubing and through the Christmas tree into a flowline.
When cementing each pipe the normal practice is to calculate the amount of cement needed, based on the annular space and the length of the space designed to be filled. However, it is often difficult to calculate the exact amount of cement needed and the cement level may be lower than intended. In the case of surface casing it is desirable to fill the annular space all the way up to the mudline (seabed), but this may not always be achieved, leading to so-called cement shortfall. The top of the surface casing may therefore be filled with a fluid (water or brine) instead of cement resulting in that the surface casing string is not bonded to the conductor pipe all the way up to the mudline. In such a case the part of the surface casing that is not cemented can be regarded as a free-standing column that, if subjected to loads, can be damaged.
The surface casing carries a wellhead and is the principal load-carrying structure for the equipment mounted on top of the wellhead. It serves both the purpose of being a foundation for external loads, such as production equipment (Christmas tree) and for borehole support against the formation. A well will be subjected to various loads during its lifetime. In for example a workover situation, a BOP and riser is attached to the Christmas tree, the riser extending to the surface. The movements of the riser and the use of drilling equipment can set up cyclic loads in the wellhead and the surface casing string (See FIGS. 1 & 2). This may induce fatigue in the casing string.
Another cause of loads comes from the casing strings being subjected to loads from being heated by the producing fluids.
If the cement has filled the annular space completely and, in addition, has bonded properly with the steel pipe cyclic loads will be spread along the length of be casing and transferred to the conductor pipe and the ground. However, if there is a length that has not been properly filled that part of casing can act as a free-standing column (ref. above) and cyclic loads can lead to fatigue and damage of the casing. It is also possible that the point where the top of cement level is can act as a breaking point because of the movements of the column above.
Similarly, heating and cooling of the casing may induce loads that can lead to fatigue problems and deformation of the casing.
As can be understood from the above it is therefore of prime interest to find out if the cement job is properly executed, e.g. the annular space is properly filled. The main purpose of the invention is therefore to find out if there is cement between the surface casing and the conductor and especially the level of the cement from which the length of the column can be determined.
If later work has to be performed on the well the BOP and riser is reattached to the Christmas tree so that operations can be carried out in a safe manner.
Both during drilling and (if necessary) workover operations the wellhead is subjected to external loads, as explained above. How this affects the wellhead depends on the length of the free standing column. A longer column will be more vulnerable to fatigue. If the length of the free standing column can be determined it can be calculated how much load the wellhead can be subjected to and this will in turn determine how much work that can be done. This enables an operator to predict the operational lifetime of the well and to ensure the integrity of the well structure.
There are known several methods and systems for non-destructive inspection of layers of different materials. Some of these methods are based on first generating electric, electromagnetic or acoustic signals and thereafter measure the different reflections of the generated signals from the layers. These methods and systems have not been used for inspection of subsea wells. One reason for this is the difficult access to the well through the central pipe. Moreover, the well represents a challenging measuring environment due to presence of hydrocarbon fluids and/or water, the temperature and pressure may vary widely and/or the central pipe may be exposed to significant clogging and corrosive wear.
There is also a challenge to create a signal that is strong enough to penetrate through several different layers and to read signals that have been reflected from the various layers. As is readily understood, the further away the reflections are the weaker the signals will be.
The object of the invention is therefore to provide a Method and apparatus for determining the nature of a material in a cavity between one inner metal wall and one outer metal wall Moreover, it is an object of the invention to provide a method and system where a tool may be lowered into the well inside the inner metal wall, for example the production tubing, for performing the operation.
There is also an object of the invention to determine the level of the cement, i.e. the depth of the top of cement and from that calculate the length of the free standing column.